Control system for fuel burners



y 8, 1934. E. M. MILLER ,958,093

CONTROL SYSTEM FOR FUEL BURNERS Filed July 28, 1932 K I U 1 EBA/E5 r /7 mus/a Patented "May 8, 19.34

UNITED STATES PATENT oF cE 1,958,093 I CONTROL SYSTEM FOR FUEL BUBNERS Ernest M. Miller, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company; Minneapolis, Minn., a corporation of Delaware Application July 28, 1932, Serial No. 625,550

16 Claims. (01. 158-28) The present invention is an improvement upon the system disclosed and claimed in my co-pend ing application Serial No. 448,011.

'This invention relates to an improved and simplifled automatic control system for heating apparatus in which there is provided a post-ignition periodthat is timed from the establishment of combustion as indicated by the operation of a combustion responsive switch. The invention further provides a standby period between operations of the fuel supply controlling device and this standby period is measured from the cessation of combustion as indicated by the operation of the combustion responsive switch. The invention also provides means for recycling the system upon a flame or power failure, an immediate shut-down upon flame failure, and a complete shut-down requiring manual intervention if the system'does not operate correctly.

The primary object of the invention is the provision of a simplified and more economical control system of the type above described.

Specifically the invention provides for the elimination of one of the relays used in former control systems of the same general type by the use of a switch which is coniointly controlled by the action of two relays and through other refinements and improvements.

For a more "complete understandingv of the invention, reference may be had to the following description and accompanying single drawing which is a diagrammatic showing of one form of the invention.

Referring to the drawing, an electrical fuel supply controlling device is herein diagrammatie cally shown as a rotating electrical motor indicated .at 1. Electrically controlled ignition means 2 are provided for igniting the fuel supplied by the motor 1. I s I A main control or room thermostat is indicated at 3. The room thermostat comprises a bimetallic element 4 which is secured at one end as indicated at 5. The free end of bimetallic element 4 carries flexible blades 6 and '7 which sequentially engage contacts 8 and 9 in the ordernamed upon a fall in room temperature. Upon a rise in room temperature, blade '7 first disengages contact 9 and then blade 6 disengages contact 8.

A first relay generally indicated at 10 comprises a coil 11 and an armature 12 which operates switch arms 13, 14, and 15. Upon energization of coil 11 armature 12 is attracted and in turn moves switch arms 13. 14, and 15 into arms 13, 14, and 15 move by gravity to the posi-' tion shown in the drawing. A second relay generally indicated at 19 comprises a coil 20 and an armature 21 which controls switch arms 22, 23, and 24. Switch" arm 22 is in engagement with a contact 25 'when the relay coil 20 is deenergized and moves out of engagement therewith uponenergization of coil 20. Likewise, switch arms 23 and 24 are moved into engagement with contacts 26 and 2'? when relay coil 20 is energized and move out of engagement therewith by the action of gravity upon deenergization of relay coil 20.

A base member partially shown at 28 supports a pair of brackets 29 and 30. A flexible switch arm 31 has one of its ends secured to bracket 29 and its free end is provided with a downwardly bent cam surface 32 and a horizontally extending portion 33. Flexible switch arm 31 carries a com tact 34 on its lower side and intermediate its ends. A similar switch arm 35 has one of its ends secured to bracket 30 and carries a contact 36 on the upper portion of its free end which contact in normally out of engagement with contact 34. Armature 12 carries an actuator 37 which normally oven-lies the horizontal portion 33 of the flexible switch arm 31., Upon energizationof relay coil 11 armature 12 is moved to bring switch arms 13, 14, and 15 into engagement with contacts 16, 17, and 18'as above described and further moves actuator 3'? into engagement with cam surface 32 which results in downward movement of flexible switch arm 31 and engagement of contact 34 with contact 36. Armature 21 is provided with an actuator 38 which passes below horizontal portion 33 upon energization of relay coil 20 if contacts 34 and 36 are separated. However,

when portion 33 of flexible" switch arm 31 is bent portion of actuator 38 will engage the upper side of portion 33, upon energization of relay coil 20, to maintain contacts34 and 36 in engagement after relay coil 11 has been de-energized. In other words, the switching mechanism constituted by contacts 34 and 36 and the associated parts can be moved to closed circuit position only upon energization of relay 10; but this switching mechanism is maintained in closed position during energization of relay 19. V

The present system also includes an electrothermal safety switch or time limit cut-out generally indicated at 39 which is herein engagement with contacts 16, 1'7. and 18. When coil 11 is deenergized armature 12 and its switch d g t ca y Show}! as m p 8 Pi l 01 contacts 40 and 41 respectively carried by flexible arms 42 and 43. The flexible arm 42 is normally biased to cause disengagement of contact 40 from contact 41, but these contacts are normally held in engagement by the end of a bimetallic element 44 which normally engages flexible blade 42. The other end of bimetallic blade 44 is secured to a similar bimetallic element 45 which in turn is held in fixed position by a bracket 46. An electrical heating element 47 is placed so as to heat bimetallic element 45. The bimetallic elements 44 and 45 have their most active faces disposed in such a manner that the free end of bimetallic element 44 will not move upon changes in ambient temperature. When current has flowed through electrical heating element 47 for a predetermined time the free end of bimetallic element, 44 will disengage the free end of flexible blade 42 and allow contact 40 to move away from contact 41. After such action it is necessary to allow bimetallic element 44 to cool after which contacts 40 and 41 must be re-engaged manually and locked in their engaged position by the free end of bimetallic element 44. 25'

The present system also utilized an electrical time switch generally indicated at 48 which comprises a bimetallic element 49 secured at one end as indicated at 50. Another bimetallic element 51 has one of its ends secured to the free end of bimetallic element 49 and the free end of bimetallic element 51 carries a pair of flexible blades 52 and 53 which normally are in engagement with contacts 54 and 55. An electric heater 56 is provided for heating bimetallic element 49 which then moves in such a direction as to cause blades 52 and 53 to disengage their co-operating contacts, these blades being so arranged that blade 53 will first disengage contact 55 after which blade 52 will disengage contact'54. The bimetallic blade 51 has its most active face reversed in respect to the most active face of bimetallic element 49 in order to compensate for variations in ambient temperature.

A combustion responsive switch is herein shown as comprising a bimetallic strip 57 which when heated as a result of burning of the fuel moves into engagement with a contact 58. The combustion responsive switch is preferably of the well-known type which operates upon the reversal of temperatures rather than at definite temperature settings although it is to be understood that other types of combustion control switches that do not respond directly to the temperature of combustion may also be utilized.

The primary 59 of a step-down transformer having a low voltage secondary 60 is connected to line wires 61 and 62 by conductors 63 and 64. The various other circuit connections for the device as above described will be fully explained under Operation.

Operation With the parts as shown in the drawing the room is warm, the temperature to which the combustion responsiveswitch responds has lowered so that element 57 is out of engagement with contact 58 and the electric heater 56 has been deenergized for a sufiicient length of time that blades 52 and 53 are in engagement with contacts 54 and 55.

As the room temperature falls, blade 6 of the room thermostat will first engage contact 8, but no circuits will be established. Upon a further fall in room temperature, blade '7 will engage contact 9 establishing an energizing circuit for electrical heating element 47 and relay coil 11 as follows: secondary 60, wire 65, contacts 41, 40, wire 66. contact 8, blade 6, blade 7, contact 9, wire 67,

'switch arm 22, contact 25, wire 68, contact 55,

blades 53, 52, contact 54, wire 69, wire '70, electrical heating element 47, wire '71, relay coil 11, and wire 72 to secondary 60. Energization of relav coilll attracts armature 12 which in turn moves switch arms 13, 14, and 15 into engagement with contacts 16, 17, and 18 and also causes closure of contacts 34 and 36 by means of actuator 3'7 and camsurface 32. Engagement of switch arm 13 with contact 16 establishes a holding circuit for relay coil 11 which is as follows: secondary 60, 65, 41, 40, 66, 8, 6, element 4, wire 73, wire 74, contact 16, switch arm 13, wire '75, bimetallic elements 49 and 51 and blade 52, contact 54, 69, '70, 4'7, 71, relay 11, and 72 back to secondary 60. It will be noted that the circuit for relay coil 11 having been once established is not interrupted until blade 6 leaves contact 8 of room thermostat 3 and blade 52 leaves contact 54 of timer 48 unless, of course, safety switch 40-41 should shut down the entire system. Engagement of switch arm 15 with contact 18 establishes a circuit for motor 1 as follows: line 61, wire 76, wire 77, switch arm 15, contact 18, wire '78, wire '79, motor 1, and wire to line 62. Likewise, engagement of switch arm 14 with contact 1'7 establishes a circuit for ignition means 2 as follows: line 61, 76, 77, wire 81, switch arm' 14. contact 17, wire 82, ignition means 2, and wire 83 to line 62.

Fuel is now being supplied to the burner by means of motor 1 and should be ignitedv by ignition means 2. Current is flowing through electrical heating element 47 and if by any chance combustion should not be established within a predetermined time, the free end of bimetallic element 44 will disengage arm-42 allowing contacts 40 and 41 to open thereby shutting down the system. Assuming, however, that combustion does take place, 5'7 will respond to the increase in temperature and move into engagement with contact 58 to establish an energizing circuit for electric heater 56 and relay coil 20 as follows; secondary 60, 65, 41, 40, 66, 8, 6, 4, 73, wire 84, element 5'7, contact 58, wire 85, electric heater 56, wire 86, relay coil 20, wire 8'7, and '72 to secondary 60. The energizing circuit for relay coil 20 and electric heater 56 does not include blade '7 and contact 9 so that relay 19 will be energized even though the room temperature has been partially restored causing blade 7 to disengage contact 9. Energization of relay coil 20 attracts armature 21 which moves switch arm 22 out of engagement with contact 25, switch arms 23 and 24 into engagement with contacts 26 and 27, and actuator 38 into a position to maintain contacts 34 and 36 closed after relay coil 11 is deenergized.

Disengagement of switch arm 22 from contact 25 breaks the initial energizing circuit for relay coil 11, but does not effect the holding circuit thereof. Engagement of switch arm 28 with contact 26 establishes a shunt circuit around electrical heating element 4'7 which is as follows: 1

lows: line 61, 76, wire 90, contacts 34, 36, wire 91, contact 2'7, switch arm 24, wire 92, 79, motor 1, and 80 to line 62.

Ignition means 2 is still operating since relay coil 11 is energized but after an interval of time the heat generated by electric heater 56 will cause bimetallic element 49 to move blades 52 and-53 out of engagement with contacts 54 and 55 which action will deenergize relay 11, break'the holding circuit thereto at 13-16, break the ignition circuit at 14-17, and break the first motorcircuit at 1518. The system is now in running position and will continue in operation till the room thermostat becomes satisfied or unless such operation is terminated by a flame orpower failure. In the event there is no flame or power failure and the room thermostat'becomes satisfied causing blade 7 to leave contact 9 and the blade 6 to leave contact 8 relay, coil will be deenergized allowing armature 21 and the associated parts to move to the pos'tion shown in the drawing. Such movement of armature 2i and the associated parts breaks the second motor circuit at 2427 and 3436, breaks the shunt circuit for electrical heating element 47 at 2326, and

partially establishes the initial energizing circuit for relay coil 11 at 22-25. If for any reason the room thermostat should immediately reclose its contacts before element 57 disengages contact 58, relay coil 20 would be energized, but the cirsuit for motor 1 would 3110i] be closed since actuator 38 is not capable of closing contacts 3436.

It the room thermostat should reclose its contacts after element 57 disengages contact 58 due to cessation of combustion, it will be noted that relay coil 11 can not be energized until bimetallic element 49 has cooled sufiiciently to cause engagement of blade 52 with contact 54 and the blade 53 with contact 55. In this manner a definite timed standby period is provided between operations of the control system.

With the system in operation if there should be a flame failure, element 5'7 wll substantially immediately disengage contact 58 to deenergize relay coil 20 and electric heater 56. The system will therefore be shut down and after bimetallic element 49 has cooled sufficiently to bring blades 52 and 53 into engagement with contacts 54 and 55, the system will be recycled provided the room is still cold.

If while the system is in operation there should be a power failure of sufiicient duration to allow element 57' to disengage element 58, then the system will recycle'upon the resumption of power or upon engagement of blades 52 and 53 with contacts 54 and 55 depending upon which of these actions is the last to take place. Shpuld the power failure be momentary, however, so that power is resumed before element 57 disengages 58fthen relay coil 20 will be energized, but the motor circu't will remain open since actuator 38 can not close contacts 34-36; all as above described, upon a reclosure of the room thermostat before opening of the combustion responsive switch contacts;

From the foregoing it will be seen that this invention provides a complete fuel burner system including a timed post-ignit'on period, and a timed standby period both of which are timed by an electrically operated timing mechanism together with an. immediate shut-down upon flame failure and a recycle upon flame or power failure. All of th's has been accomplished by the novel circuit arrangement which includes the switching mechanism -34-436 which is con trolled by the conjoint action of relays 10 and 19.

While a" specific embodiment oi the invention has been herein shown and described, it is to be understood that I am to be limited only in the purview of the appended claims.

I claim as my invention:

1. A control system .for supplying fuel to a burner comprising, an electrical device for controlling the'supply of fuel to theburnr, electrically controlled igniton means forigniting the fuel supplied to the burner, switching means in circuit with said device and ignition means, a first relay for operating said switching means, an electr'cal timing device, a timer operated swltch, a main control, a circu't for the first relay including the main control and timer operated switch, a second relay, a switching mechan'sm operated by the conjoint action of the first and second relays, a second circuit for the fuel supply controlling device including the switching mechanism, a combustion responsive switch, and a circuit arrangement controlled by the main control and combustion responsive switch for simultaneously energizing the second relay and the electrical t'm'ng device.

2. A control system for supplying fuel to a burner comprising, an electrical device for controlling the supply of fuel to the burner, electrically-controlled ignition means for igniting the anism, a combustion responsive switch, and a circut arrangement controlled by the main control and combustion responsive switch for simultaneously energizing the second relay and the electrical timing device.

3. A control system for supplying fuel to a burnercomprising, an electrical device for controlling the supply of fuel to, the burner, electrically controlled ignition'means for ignitingihe fuel supplied to the burner, switching means in circuit with said device and igniting means, a first relay for operating said switching means, an electrical, timer comprising a thermostatic switch and an electrical heating element therefor, a main control, a circuit for the first relay controlled by the main control and thermostatic switch, a second relay, a switching mechanism operated by the conjoint action of the first and second relays, a second circuit for the fuel supply controlling device controlled by said switching mechanism, a combustion responsive switch, and an energizing circuit for the second relay and electrical heating elementof the electrical timer controlled by the main control and combustion responsive switch.

4. A control system for supplying fuel to a burner comprising, an electrical device for controlling' the supply of fuel to the burner, electrically controlled ignition means for igniting the fuel supplied to the burner, switching means in circuit with said device and ignition means, a first relay'for operating said switching means, an electrical timer comprising a thermostatic switch and an electrical heating element therefor, a main control, a circuit for the first relay controlled by the main control and thermostatic energized and held in closed position by the second relay when energized, a second circuit for the fuel supply controlling device controlled by said switching mechanism, a combustion responsive switch, and an energizing circuit for the second relay and electrical heating element of the electrical timer controlled by the main control and combustion responsive switch. K

5. A control system for supplying fuel to a burner, comprising, an electrical fuel supply controlling device for supplying fuel to the burner,

an electrically controlled ignition means for igniting the fuel supplied to the burner, a first relay, a second relay, a switching mechanism moved to closed position by the first relay when energized and held in closed position by the second relay when energized, said second relay being unable to move said switching mechanism from open to closed position, first and second switches operated by said first relay, a circuit for the ignition means controlled by said first switch, a circuit for the fuel supply controlling device controlled by said second switch, a parallel circuit for said fuel supply controlling device controlled by said switching mechanism, a main control, a circuit for the first relay including the main control, a combustion responsive switch, a circuit for the second relay controlled by the main control and combustion responsive switch, and means controlled by the combustion responsive switch for operatively deenergizing the first relay as a result of the establishment of combustion.

6. A control system for supplying fuel to a burner, comprising, an electrical fuel supply controlling device for supplying fuel to the burner, an electrically controlled ignition means for igniting the fuel supplied to the burner, a first relay, a second relay, a switching mechanism moved to closed position by the first relay when energized and held in closed position by the second relay when energized, said second relay being unable to move said switching mechanism from open to closed position. first and second switches operated by said first relay, a circuit for the ignition means controlled by said first switch, a circuit for the fuel supply controlling device controlled by said second switch, a parallel circuit for said fuel supply controlling device controlled by said switching mechanism, a main control, a time limit cut-out including a thermostatically controlled resettable switch and an electric heater, a circuit for the first relay and electric heater including the main control and resettable switch, a combustion responsive switch, a circuit for the second relay including the main control, resettable switch and combustion responsive switch,- and means controlled by the combustion responsive switch for op'eratively deenergizing the heater coil and the first relay if combustion is established.

7. A system-of the class described, comprising, an electrical fuel supply controlling device, electrically controlled ignition means, a first relay, energizing circuits for the fuel supply controlling device and ignition means controlled by said first relay, a second relay, a switch operated thereby,

switching mechanism, connections between the first relay and switching mechanism for ,moving the same to closed position when the first relay is energized, connections between the second relay and switching mechanism for holding the same closed but incapable of closing the switching mechanism when the second relay is energlzed, a second energizing circuit for the fuel switch, a main control, a circuit for the first an electrical fuel supply controlling device, electrically controlled ignition means, a first relay, energizing circuits for the fuel supply controlling device and ignition means controlled by said first relay, a second relay, a switch operated thereby, switching mechanism, connections between the first relay and switching mechanism for moving the same to'closed position when the first relay is energized, connections between the second relay and switching mechanism for holding the same closed but incapable of closing the switching mechanism when thesecond relay is energized, a second energizing circuit for the fuel supply controlling device controlled by said second relay operated switch and the switching mechanism, an electrical timer, a timer operated relay controlled by the main control and timer operated switch, a combustion responsive switch, and a circuit for the second relay and electrical timer controlled by the main control and coinbustion responsive switch.

9. A system of the class described, comprising, an electrical fuel supply controlling device, electrically controlled ignition means, a first relay, energizing circuits for the fuel supply controlling device and ignition means controlled by said first relay, a second relay, a switch operated thereby, switching mechanism, connections between the first relay and switching mechanism for moving the same to closed position when the first relay is energized, connections between the second relay and switching mechanism for holding the same closed but incapable of closing the switching mechanism when the second relay is energized, a second energizing circuit for the fuel supply controlling device controlled by said 1 0 second relay operated switchand the switching mechanism, a time limit cut-out comprising a. thermostatically operated resettable switch and an electric heater for the thermostat thereof, an electrical timer, a timer operated switch, a main control, a circuit for the first relay and electric heater controlled by the main control, resettable switch and timer operated switch, a combustion responsive switch, a circuitfor the second relay and electrical timer controlled by the main control, resettable switch, and combustion responsive switch, and means controlled by the second relay when energized for operatively deenergizing the electric heater.

10. A system of the class described, comprising, an electrical fuel supply controlling device, electrically controlled ignition means, a first relay, energizing circuits for the fuel supply controlling device and ignition means controlled by said first relay, a second relay, 9. switch operated thereby, switching mechanism, connections between the first relay and switchingmechanism for moving the same to closed position when the mechanism, a time limit cut-out comprising a thermostatically operated resettable switch and an electric heater in series with the first relay, an electric timer, a timer operated switch, a main control, an energizing circuit for the electric heater and first relay controlled by the main control, resettable switch and timer operated switch, a

a combustion responsive switch, a second switch moved to closed position by the second relay when energized, a circuit for the second relay and electrical timer controlled by the main control, resettable switch and combustion responsive switch, and a shunt circuit for tlli electric heater controlled by the second switch operated by the second relay.

11. A system of the class described, comprising, an electrical fuel supply controlling device, electrically controlled ignition means,'a first relay, energizing circuits for the fuel supply controlling device and ignition means controlled by said first relay, a second relay, a switchoperated thereby, switching mechanism, connections between the first relay and switching mechanism for moving the same to closed position when the first relay is energizedconnections between the second relay and-switching mechanism for holding the same closed but incapable of, closing the switching mechanism when the second relay is energized, a

, second energizing circuit for the fuel supply controlling device controlled by said second relay operated switch and the switching mechanism, an electrical timer in series with the second relay, a

' timer operated switch, a maincontrol, a circuit for the first relay controlled by the main control and timer operated switch, a combustion responsive switch, and a circuit for the second relay and electrical timer including the main control and combustion responsive switch in series.

12. A system of the class described, comprising, an electrical fuel supply controlling device, electrically controlled ignition means, energizing circuits for thefuel supply controlling device and ignition means, a first relay in control thereof, a second relay, switching mechanism operated by the conjoint action of the first and second relays, a second circuit, for the fuel supply controlling device controlled by said switching mechanism, an electrical timer, a timer operated switch, a main control, 'a switch controlled by the second relay and closed when therelay is deenergized, an energizing circuit for the first relay controlled by the main control, the timer operated switch and the second relay operated switch in series, a holding-circuit for the, first relay controlled by the main control'and timer operated switch but independent of the second relay opei'ated switch, a combustion responsive swit h, and a circuit for the second relay and electrical timer controlled by the main control and the combustion responsive switch in series.

13. A system of the class described, comprising, an electrieal-fuel supply controlling device, electrically controlled ignition means, energizing circuits for the fuel supply controlling device and ignition means, a first relay in control thereof, a second relay, switching mechanismoperated by the conjoint action of the first and second relays, a second circuit for the fuel supply controlling device controlled by said switching mechanism, an electrical timer, a timer operated switch, a main control, a. switch controlled by the second relay and closed when the relay is deenergized, a time limit cut-out comprising a thermostatically controlled resettable switch and an electric heater, an energizing circuit for thefirst 'the secondswitch and the thermostatic member ber of the timer and one of its contacts, a comelectrically operated ignition means, a first relay,

a. firstswitch and a switching device controlled thereby, energizing circuits for the fuel supply controlling device and ignition means controlled by said switching device, a second relay, a second switch controlled thereby, a switching mechanism moved to closed position by the first relay when energized and maintained in closed position by the second relay when energized, a room thermostat comprising a movable element and two sta tionary contacts sequentially engageable' thereby on temperature fall, a timer comprising a thermostatic element, two contacts which are sequentially disengaged on heating, and an electric heater, an energizing circuit for the first relay comprising the movable element of the room thermostat and both of its stationary contacts,

of the timer and both its contacts, a holding circult for the first relay comprising the movable element of the room thermostat and one of its contacts, the first switch, and the thermostatic memhi6 bustion responsive switch, and a circuit for the second relay and electric heater comprising the movable element of the room thermostat and one of its contacts, and the combustion responsive switch. v V

15. A system of the class described, compris ing an electrical fuel supply controlling device, an electrically operated ignition means, a first relay, a first switch and a switching device controlled thereby, energizing circuits for the fuel supply controlling device and ignition means controlled by said switching device, a second relay, 9. second switch controlled thereby, a switching mechanism moved to closed position by the first relay when energized and maintained in closed 5 position by the second relay when. energized, a

room thermostat comprising a movable element and two stationary contacts sequentially engage-.

able thereby on temperature fall, a timer comprising 'a thermostatic element, two contacts 1 which are sequentially disengaged on heating, and an electric heater, a time limit cut-out comprising a thermostatically operated resettable switch and an electrical heating element, an energizing circuit for the first relay and electrical heating element comprising the movable element of the room thermostat and both its contacts, the second switch, the thermostatic member of the timer and both-its contacts, and the resettable switch, a holding circuit for the first relay comprising the movable element and one of its contacts, the first switch, the thermostatic member of the timer and one of its contacts and the resettable switch, a combustion responsive switch, an energizing circuit for the second relay and electric heater of the timer comprising the movable element of the room thermostat and one of its contacts, the resettable switchand the combustion responsive switch, and means controlled by for operatively deenergizing the electrical heating element of the time limit cut-out.

16. A system of the class described, comprising, an electrical fuel supply controlling device, an electrically controlled ignition means, a first relay, first, second and third switches operated from open to closed position by, the first relay when energized, a second relay, fourth and fifth switches operated from open to closed position by the second relay when energized and a sixth switch operated from closed to open position by the second relay when energized, a switching mechanism moved to closed circuit position by the first relay when energized and maintained in closed position by the second relay when energized, a circuit for the fuel supply controlling device controlled by the first switch, a circuit for the ignition means controlled by the second switch, a second circuit for the fuel supply controlling device controlled by the fourth switch and said switching mechanism in series, a room thermostat comprising a movable element and two contacts sequentially engageable thereby on temperature fall, a timer comprising a thermostatic member, two

contacts sequentially disengageable thereby on heating and an electric heater coil in series with the second relay, a time limit cut-out comprising a thermostatically operated resettable switch and an electrical heating element in series with the first relay, an energizing circuit for the first relay and the electrical heating element of the time limit cut-out comprising the movable element and both the contacts of the room thermostat, the thermostatic element and both the contacts of the timer, the resettable switch, and the sixth switch, a holding circuit for the first relay and the electrical heating element controlled by the movable element and one contact of the room thermostat, the thermostatic member and one of the contacts of the timer, the third switch and the resettable switch, a combustion responsive switch, an energizing circuit for the second relay controlled by the movable element and one contact of the room thermostat, the combustion responsive switch and the resettable switch, and a shunt circuit for the electrical heating element of the time limit cutout controlled by the fifth switch.

ERNEST M. MILLER. 

